1. Field of the Invention
The present invention relates to a navigation system for automotive vehicles and, more particularly, to an automotive vehicle navigation system which controls vehicle speed as the vehicle makes a turn.
In return years, various kinds of automatically controlled electronic driving systems have been developed which enable drivers to drive, safely and comfortably, independently of their particular feelings and driving techniques. For example, it is known to have vehicle distance control systems automatically decelerate an automotive vehicle, as well as provide an alarm, when the automotive vehicle is closer to a leading vehicle than a safe distance corresponding to a vehicle speed at which the automotive vehicle travels. Such a vehicle distance control system is known from, for instance, Japanese Unexamined Patent Publication No. 57(1982)-155,700. The vehicle distance control system in the publication mentioned above detects its own vehicle speed and the distance between a leading vehicle and compares the distance with what is considered a safe distance corresponding to the vehicle speed.
2. Description of the Related Art
For safe driving, vehicles must be controlled to be held at a safety speed which depend on road conditions as well as the distance between two vehicles. As an example, in negotiating a sharp turn, when a vehicle, traveling on a straightway at a relatively high speed, enters into a sharp turn, it should be brought down to a proper approaching speed, lower than the traveling speed, corresponding to a radius of the sharp turn before an approach point of the turn. If the vehicle enters into the sharp turn at a speed higher than the proper approaching speed, it possibly will swerve from the sharp turn, due to a centrifugal force which is proportional to the radius of the turn and varies inversely with the square of vehicle speed. However, if a turn in the vehicle path is not visible, it is difficult to estimate the radius of the turn. Moreover, although a turn in the path may be visible enough to estimate the radius of the turn, if the path is icy or wet, safe driving is assured only when a vehicle enters into the turn at a speed lower than a speed visually estimated corresponding to the radius of the turn. Such a radius and speed estimation is hard to make accurately and quickly during driving.